﻿/**
 * Copyright (c) 2013 Jeremy Wyman
 * Microsoft Public License (Ms-PL)
 * This license governs use of the accompanying software. If you use the software, you accept this license. If you do not accept the license, do not use the software.
 * 1. Definitions
 *    The terms "reproduce", "reproduction", "derivative works", and "distribution" have the same meaning here as under U.S. copyright law.
 *    A "contribution" is the original software, or any additions or changes to the software.
 *    A "contributor" is any person that distributes its contribution under this license.
 *    "Licensed patents" are a contributor's patent claims that read directly on its contribution.
 * 2. Grant of Rights
 *    (A) Copyright Grant - Subject to the terms of this license, including the license conditions and limitations in section 3, each contributor grants you a non-exclusive, worldwide, royalty-free copyright license to reproduce its contribution, prepare derivative works of its contribution, and distribute its contribution or any derivative works that you create.
 *    (B) Patent Grant - Subject to the terms of this license, including the license conditions and limitations in section 3, each contributor grants you a non-exclusive, worldwide, royalty-free license under its licensed patents to make, have made, use, sell, offer for sale, import, and/or otherwise dispose of its contribution in the software or derivative works of the contribution in the software.
 * 3. Conditions and Limitations
 *    (A) No Trademark License - This license does not grant you rights to use any contributors' name, logo, or trademarks.
 *    (B) If you bring a patent claim against any contributor over patents that you claim are infringed by the software, your patent license from such contributor to the software ends automatically.
 *    (C) If you distribute any portion of the software, you must retain all copyright, patent, trademark, and attribution notices that are present in the software.
 *    (D) If you distribute any portion of the software in source code form, you may do so only under this license by including a complete copy of this license with your distribution. If you distribute any portion of the software in compiled or object code form, you may only do so under a license that complies with this license.
 *    (E) The software is licensed "as-is." You bear the risk of using it. The contributors give no express warranties, guarantees or conditions. You may have additional consumer rights under your local laws which this license cannot change. To the extent permitted under your local laws, the contributors exclude the implied warranties of merchantability, fitness for a particular purpose and non-infringement.
 *    
 * More info on: http://tempest.codeplex.com
**/

using System;
using System.Collections;
using System.Collections.Generic;

namespace Tempest.Collections
{
    /// <summary>
    /// Represents an object in memory that may or may not be loaded into memory.
    /// </summary>
    /// <typeparam name="TKey">The type of the key in the object.</typeparam>
    /// <typeparam name="TValue">The type of the value in the object.</typeparam>
    public sealed class Lazy<TKey, TValue> : IEquatable<Lazy<TKey, TValue>>
    {
        #region Constructors
        /// <summary>
        /// Initialized a new instance of Isris.Lazy, where the value may or may not be present in memory.
        /// </summary>
        /// <param name="key">A specific key which represents the value.</param>
        /// <param name="func">The function used to retrieve a TValue for a given TKey.</param>
        public Lazy(TKey key, Func<TKey, TValue> func)
        {
            #region Argument Validation
            if (func == null)
                throw new ArgumentNullException("func", "The function pointer to load the value cannot be null");
            #endregion
            _key = key;
            _value = default(TValue);
            _func = func;
            _have = false;
        }
        #endregion
        #region Members
        /// <summary>
        /// Gets the Key used to represent the value object.
        /// </summary>
        public TKey Key { get { return _key; } }
        /// <summary>
        /// Gets or sets the value of the object.
        /// </summary>
        /// <remarks>Accessing this member can cause the loading of the value into memory.</remarks>
        public TValue Value
        {
            get
            {
                // use the function pointer to lock the object to add a little thread safety
                lock (_func)
                {
                    if (!_have)
                    {
                        _value = _func(_key);
                    }
                }
                return _value;
            }
            set
            {
                // use the function pointer to lock the object to add a little thread safety
                lock (_func)
                {
                    _value = value;
                    _have = true;
                }
            }
        }
        #region Private Members
        private TKey _key;
        private TValue _value;
        private Func<TKey, TValue> _func;
        private bool _have;
        #endregion
        #endregion
        #region Methods
        /// <summary>
        /// Indicates whether the current object is equal to another object of the same type.
        /// </summary>
        /// <param name="that">A Lazy object to compare with this Lazy object.</param>
        /// <returns>true if the current object is equal to the other parameter; otherwise, false.</returns>
        bool IEquatable<Lazy<TKey, TValue>>.Equals(Lazy<TKey, TValue> that)
        {
            return this._key.Equals(that._key);
        }
        #endregion
    }
}
